1. Field of the Invention
The present invention relates to an objective lens used in an optical pickup for recording information signals to and/or reproducing information signals from three different types of optical discs. The present invention also relates to an optical disc apparatus including the optical pickup.
2. Description of the Related Art
In recent years, an optical disc in a next-generation optical disc format that allows high-density recording and reproduction of signals using a light beam having a wavelength of about 405 nm emitted from a blue-violet semiconductor laser has been proposed (hereinafter referred to as a “high-density-recording optical disc”). Such a high-density-recording optical disc has a thin cover layer for protecting a signal recording layer. The cover layer has a thickness of, for example, 0.1 mm.
When producing an optical pickup for such a high-density-recording optical disc, it is desirable that the optical pickup be compatible with existing optical discs. That is, it is desirable that the optical pickup be compatible with existing optical discs in different formats such as the compact disc (CD) format using a wavelength of about 785 nm and the digital versatile disc (DVD) format using a wavelength of about 655 nm. Thus, it is desirable that the optical pickup and an optical disc apparatus including the optical pickup be compatible with optical discs in different formats having different disc structures and adapted to different laser specifications.
An existing method for recording information signals to and reproducing information signals from the optical discs in three different formats uses two types of objective lenses. To be specific, an optical pickup using the method includes two types of objective lenses and optical systems, one for DVDs and CDs and the other for high-density-recording optical discs. The optical pickup switches between the objective lenses in accordance with the used wavelength.
However, because the optical pickup has two objective lenses, each of the objective lenses may not be mounted on a lens holder of an actuator at an appropriate angle. Therefore, the angles between the objective lens and the optical disc may not be optimal, so that the quality of a reproduced signal may be reduced. The optical pickup has a large number of components because it is necessary to provide optical components to the optical paths of two optical systems. Therefore, the production cost and the size of the optical pickup increase. Moreover, the weight of the actuator increases because it is necessary to mount two objective lenses to the actuator for driving the objective lenses. Therefore, the sensitivity of the optical pickup is reduced.
To address such problems and to simplify the optical components, optical pickups including a single objective lens compatible with a plurality of types of optical discs and compatible with three used wavelengths are being developed. The basic idea behind using a single objective lens for three wavelengths is to provide a diffraction section on the optical path and diffract light with the diffraction section in a divergent direction or in a convergent direction, so as to correct spherical aberration generated in accordance with a combination of a wavelength and a medium.
However, such optical pickups that have been developed, which use a single objective lens to achieve three-wavelength compatibility, do not take into account an increase in unwanted diffracted light generated by a diffractive structure, which is caused by variations in temperature and wavelength. The problem is that, because the unwanted light forms an image on a light-receiving element in the same manner as the normal light, a signal from another layer leaks into a focus servo signal and thereby focus servo control is not stably performed in particular when reproducing information signals from a double-layer optical disc (Japanese Unexamined Patent Application Publication No. 2004-265573).